feat(theme): runtime custom theme ecosystem (graphics-only)

Runtime-loaded visual re-skin system (no client rebuild, real themes never
hit git). A theme = a folder on the server (theme.base.url) with a manifest +
CSS "pieces"; each piece is toggled from Settings > Themes (checkboxes). A
broken/404 piece auto-falls back to the default (per piece). Hotel-wide default
via ui-config theme.default (+ theme.default.pieces), per-user override in
localStorage (same pattern as the catalog style toggle).

- api/theme/ThemeManager: fetch index/manifest + inject/remove <link> + fallback
- hooks/theme/useThemes: state + persist + default-from-config + live apply
- components/theme/ThemeApplier: applies on boot (mounted in MainView)
- UserSettings: General/Themes tabs with theme selector + per-piece checkboxes
- custom-themes/: reference template (demo theme "Neon Viola" + README)
- .gitignore: public/custom-themes/ (real themes are never committed)

custom-themes/neon-viola/toolbar.css

@@ -0,0 +1,9 @@
+/* Tema Neon Viola — pezzo "toolbar".
+   Best-effort: ricolora la barra strumenti in basso. Se i selettori non
+   matchano nella tua build, il pezzo non ha effetto (fallback sicuro). */
+
+.nitro-toolbar,
+[class*="toolbar-container"] {
+    background: linear-gradient(180deg, #2a0a4a 0%, #1a0730 100%) !important;
+    box-shadow: 0 -2px 10px rgba(124, 58, 237, .4) !important;
+}